1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method for producing the same. In particular, the present invention relates to an LCD device having broad viewing angle characteristics, and a method for producing the same.
2. Description of the Related Art
The inventors of the present invention have disclosed in Japanese Laid-Open Publication No. 7-120728 a display mode designated "ASM mode" (axially symmetric aligned microcell mode), in which liquid crystal molecules are allowed to be twisted across the interspace between a pair of substrates and yet be axially-symmetrically oriented within each pixel region.
This method amounts to a technique of aligning liquid crystal molecules in an axially symmetrical orientation by utilizing phase separation from a mixture of liquid crystal and a photocurable resin. This mode is a so-called normally-white mode, where the liquid crystal molecules in an axially symmetrical orientation are aligned along a normal direction of the substrates in response to an applied voltage.
This conventional ASM mode LCD device employs a liquid crystal material having a positive dielectric anisotropy .DELTA..epsilon.. Although this display mode provides excellent display characteristics in all directions because of the axially symmetrical orientation of liquid crystal molecules, it has a problem in that a black matrix having relatively large light-shielding portions must be employed in order to prevent the unwanted passage of light in the absence of an applied voltage. This has posed practical constraints on the upper limit of the aperture ratio. Furthermore, the conventional ASM mode utilizes a phase separation step, which requires complex temperature control to achieve an axially symmetrical orientation of liquid crystal molecules. In addition, it is difficult to produce a conventional ASM mode LCD device because it requires high-precision positioning of a pair of substrates, i.e., a color filter substrate and an active matrix substrate.
In view of the above problems, the inventors have proposed in Japanese Laid-Open Publication No. 8-341590 an ASM mode LCD device which is relatively easy to produce.
In accordance with the LCD device proposed in the 8-341590 application, the liquid crystal molecules within a liquid crystal layer interposed between a pair of substrates have a negative dielectric anisotropy (i.e., .DELTA..epsilon.&lt;0), and a negative phase plate having refractive index anisotropy is provided on the inside of a pair of polarizing plates. In the absence of an applied voltage, the liquid crystal molecules are aligned generally perpendicularly with respect to the substrate plane, so that a black state is attained regardless of the viewing angle. Furthermore, since the liquid crystal molecules are aligned so as to be axially-symmetrical within each pixel region, this device functions so as to compensate for the retardation which the light outgoing from the liquid crystal layer in any oblique direction is subjected to in a TN (twisted nematic) LCD device.
However, the above-described conventional LCD device has a problem in that it provides a very gentle voltage-transmittance characteristic curve so that a high voltage is incurred at the time of saturation. As a result, it is difficult to drive the conventional LCD device with a low voltage, resulting in large power consumption.
The above-described conventional LCD device attains an excellent black state in the absence of an applied voltage regardless of the direction of observation, thereby providing viewing angle characteristics with an excellent contrast ratio. However, when an intermediate gray scale image displayed on the LCD device is observed in the direction of e.g., about 45.degree. with respect to the absorption axis of a polarization plate, the contrast may greatly deteriorate, and the gray scale characteristics may even be inverted in a certain range of viewing angles (e.g., about 35.degree. to about 50.degree.) due to the retardation caused by the ASM liquid crystal cell and the characteristics of the polarization plate.
Moreover, in accordance with the LCD device proposed in the 8-341590 application, the step of attaching the pair of substrates to each other still requires highly precise positioning and use of plastic beads or the like for controlling the cell thickness, as in the case of other conventional LCD devices.